The present invention relates to differential amplifiers and, more particularly, to a linearized wide band differential amplifier and method of eliminating distortion of the output current produced therefrom responsive to an applied differential input voltage.
There are many applications which require the use of differential amplifiers in which it is not desirable to introduce non-linearities in the output signal produced therefrom. One such application is in television receivers and, in particular, the differential IF input signal to the demodulator section of the TV demodulating system. Typically, a differential amplifier is used in the TV demodulating system as the interface between the IF output stage and the demodulator. In general the IF output signal is a differential voltage that is applied to the differential amplifier. The differential amplifier converts the voltage to a differential current that is supplied to the demodulator of the TV demodulating system. A problem associated with some prior art differential amplifiers is that distortion is introduced in the current outputs thereof due to the transistors base-emitter diode non-linearities. In monaural sound TV systems this may not be a problem. However, in high quality stereo TV systems this distortion can lead to beat notes which can lie close to the sound carrier signal. If the distortion is high enough, these beat notes can become audible which is highly undesirable.
In an attempt to eliminate this distortion, the prior art has included linearizing stages coupled to the input stage of the differential amplifier. Because the linearizing stage of the prior art has a frequency response different than the direct signal path phase errors are introduced. Hence, although the prior art linearizing scheme may work quite well at low frequencies, distortion generated in the differential amplifier will not be cancelled at high frequencies due to the phase shift in the cancelling current path of the linearizing stage with respect to the direct input signal path.
Thus, a need exists for a wideband linearizing scheme for eliminating distortion in a differential amplifier.